PROJECT SUMMARY More and more US states are legalizing cannabis for medical purposes, and the growing popularity of medical cannabis has outpaced scientific understanding of its benefits and harms. The most commonly reported reason for using medical cannabis is chronic pain. However, for the vast majority of medical cannabis users with chronic pain, the proximal antecedents of cannabis use, and the health benefits and risks of use, are unknown. This is because the best available evidence that pain triggers medical cannabis use is based on retrospective reports that are subject to problems of recall bias, and the best available evidence that medical cannabis alleviates pain, with few adverse side effects, is based on clinical trials that lack external validity. Clinical trials lack external validity because the types of cannabis administered bear little resemblance to medical dispensary cannabis, and because the sterile lab environments of clinical trials bear little resemblance to real-world settings. Therefore, research on the proximal antecedents and subjective effects of medical cannabis calls for measurement in the natural ecology. This project will examine proximal antecedents and acute effects of cannabis use in medical users? natural environments. It will explore an array of possible antecedents of cannabis use among medical users, including pain but also negative affect and cannabis withdrawal ? a symptom of cannabis use disorder (CUD). Because research shows that the prevalence of CUD is elevated among medical cannabis users, research is needed to understand mechanisms that might underlie the transition from medical cannabis use to CUD. Contemporary models of addiction posit that subjective states during periods of acute abstinence and subjective drug effects are key mechanisms in addiction development and maintenance. However, it is not known if and how the proximal antecedents and subjective effects of cannabis use relate to CUD severity among medical users. If medical cannabis causes a high or alleviates pain and negative affect in the everyday lives of medical users, for example, it is important to know whether these effects are associated with CUD severity. This information will bolster the development of safer and more effective treatment options. To address gaps in existing knowledge, the current proposal will use ecological momentary assessment (EMA) in a group of young-adult medical cannabis users to examine three specific aims: (1) Characterize the proximal antecedents of cannabis use in the daily lives of young-adult medical cannabis users; (2) characterize the acute subjective effects of cannabis in the daily lives of young-adult medical cannabis users; and (3) test whether the proximal antecedents and subjective effects of medical cannabis use vary as a function CUD severity. This information will help patients, physicians, and policymakers to make evidence-based decisions about the pros and cons of whether to use/recommend cannabis for medical purposes.